Temperature-sensitive assist for temperature-controlled switch

ABSTRACT

In a conventional type of thermostat the temperature-sensing element, usually in the form of a bimetal strip, acts against a resiliently loaded switch contact, tending to move that contact to switch-open position against the action of the spring loading thereof. In most instances as the bimetal changes its shape progressively in accordance with changes in temperature, the force exerted thereagainst by the switch contact that it is tending to open will increase, thereby making the bimetal strip work harder and harder as the temperature changes, and in any event the force against which the bimetal works is comparatively great. These are sources of inaccuracy. To eliminate them an additional temperature-sensitive element is provided, one which normally is comparatively inactive on the switch. However, when a temperature is reached which is close to the desired operating temperature of the switch, that element becomes active on the switch with a force which is not strong enough in and of itself to move the switch to open position. Hence the element, when active, will act to assist the bimetal, thereby enabling the bimetal to work against a relatively low opposing force and thus increasing the accuracy with which the bimetal can control the opening and closing of the switch.

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Unite States Patent H 1 11 72,@

Levinn mlMarch E3, 1973 TEMPERATURE-SENSITIVE ASSIST [57] ABSTRACT FORTEMPERATURE-CONTROLLED In a conventional type of thermostat thetemperature- SWITCH sensing element, usually in the form of a bimetalstrip, [75] Inventor: Robert N. Levinn, Catskill, N.Y. acts against aresiliently loaded switch contact, tending to move that contact toswitch-open position against the action of the spring loading thereof.In most instances as the bimetal changes its shape [22] Filed: Dec. 29,1970 progressively in accordance with changes in tempera- I ture, theforce exerted thereagainst by the switch con- [211 Appl' 102301 tactthat it is tending to open will increase, thereby making the bimetalstrip work harder and harder as [52] US. Cl ..337/95, 337/335 thetemperature changes, and in any event the force [Sl] llnt.Cl. ..H0lh51/34 against which the bimetal works is comparatively Field Search37/95, 100, 0 36. great. These are sources of inaccuracy. To eliminate219/505 them an additional temperature-sensitive element is provided,one which normally is comparatively inac- [56] References Cit d tive onthe switch. However, when a temperature is reached which is close to thedesired operating tem- UNITED STATES PATENTS perature of the switch,that element becomes active [73] Assignee: American ThermostatCorporation, South Cairo, N.Y.

2,192,633 3 1940 Beam ..337/335 011 the swi ch with a f rce which i n rng en ugh 2,288,640 7/1942 Paulus ..337/89 in and of itself to move theswitch to open position. 3,254,180 5/196 Flanagan Hence the element,when active, will act to assist the 3,403,238 9/1968 Buehler et a1...337/l40 bimetal thereby enabling the bimetal to work against arelatively low opposing force and thus increasing the PrimaryExaminer-Bernard Gllheany accuracy with which the bimetal can controlthe open- Assistant Examiner-F. E. Bell mg and closing of the SwitchAttorney-James & Franklin 8 Claims, 2 Drawing Figures PATENTEDMAR 13I975 INVENTOR EOBEET IV. LZV/A/A/ yaw/14 ATTORNEY TEMPERATURE-SENSITIVEASSEST FOR TEMPERATURE-CONTROLLED SWITCH The present invention relatesto an improvement in thermostat constructions by means of which thenormal temperature-sensing element, when the temperature at which it isto actuate the switch is approached, is assisted in its switchingaction, thereby to make its control of the switch operation moreaccurate.

In the conventional thermostat a temperaturesensing element such as abimetal strip will change its shape gradually and progressively as thetemperature to which it is subjected changes. This bimetal strip is madeactive upon a switch contact which is normally biased strongly in adirection toward the bimetal, the bimetal, when it changes shapesufficiently, engaging that contact and tending to move it against theaction of the biasing means active on the contact. As the bimetalcontinues to bend the switch contact is moved with the bimetal againstthe action of the biasing means, and this usually results in anincreased resistance to movement on the part of the switch contact.While the bimetal is essentially a reasonably accuratetemperature-sensing instrumentality, when it acts against an appreciableforce, and especially such a force the magnitude of which is notconstant, its accuracy becomes degraded. Nevertheless, this drawback tothe use of bimetals in thermostats has been accepted in the past becauseof the necessity of providing sufficient biasing force on the switchcontacts so as to ensure that they will have the requiredcharacteristics, when the switch is closed, of low resistance and lackof susceptibility to shock and vibration.

In a standard type of thermostat, the switch is defined by a pair ofresilient leaves, the limiting position of the first of those leaves,usually moving in the same direction as the bimetal, being adjustable inorder to adjust the particular temperature at which the bimetal will beeffective to open the switch. When that limiting position is close tothe normal position of the bimetal the other spring leaf, the one thatthe bimetal operatively engages, will not have to move very far beforeits contact is separated from the contact carried by the first andposition-limited leaf. On the other hand, if the limiting position ofthe first leaf is located relatively remote from the normal position ofthe bimetal, then the bimetal will have to bend further until it hasmoved the other leaf which it engages sufficiently initially to causethe first and position-limited leaf to come to a stop and then to causethe other and bimetal-engaged leaf to move away, thereby to separate theswitch contacts. The greater the degree of movement of the other andbimetal-engaged leaf, the greater is the resilient force which that leafexperts against the movement of the bimetal. Hence the force which thebimetal will have to overcome will at all times be appreciable, and itwill also differ depending upon the particular temperature selectedwithin the range of variation permitted by a given thermostat. it wouldbe preferable if the bimetal were simply a positioning device, actingagainst no force at all, or, if it must act against force, actingagainst a small and constant force. When the force against which thebimetal must act is relatively great, and particularly when thatappreciable force will vary from one actuating temperature to another,the reliability and accuracy of the thermostat is subject to variation.

It is the prime object of the present invention to devise a thermostatassembly which avoids the abovementioned sources of error. Moreparticularly, it is a prime object of the present invention to provide abimetal in which the resilient loading of the switch con tacts may bemaximized to provide for proper permanent electrical connection, but inwhich the temperature sensing element such as the bimetal, when it isactive to actuate the switch, works against a much smaller for.ce thanthat required for optimum retention of the switch contact in closedposition.

Another object of the present invention is to devise a thermostatarrangement in which the operation of the temperature-sensing element,when the temperature range over which the device is adjustable isapproached, is assisted so that the temperature-sensing element itselfneed exert only a minimal force in order to effect its desiredswitch-actuating operation.

It is a further object of the present invention to devise a thermostathaving an actuation-assist feature within the operative temperaturerange which is simple and inexpensive, and which in particular isadapted for use in conjunction with standard thermostat constructionswhile adding only a minimal amount to the cost of those constructions,

For convenience of explanation the temperaturesensing element willhereinafter be termed bimetal, it being understood thattemperature-sensing elements other than bimetals could be employed,provided that they, like bimetals, have the property of substantiallycontinuously changing their shape as the temperature to which they aresubjected varies.

In accordance with the present invention a mechanical assist to thenormal operation of the bimetal is provided by an element which istemperature-sensitive in a different way from the bimetal. Such aselement will normally retain its shape without any appreciable changeover a significant range of temperatures, but when a particulartemperature has been reached, that element will tend to change its shapequite radically and exert an appreciable amount of force in thus tendingto change its shape. One substance having that characteristic is anickel-titanium intermetallic compound known as Nitinol." it isdisclosed in U.S. Pat. No. 3,174,851 of Mar. 23, 1965, entitledNickel-Base Alloys," U.S. Pat. No. 3,351,463 of Nov. 7, 1967 entitledHigh Strength Nickel-Base Alloys," and U.S. Pat. No. 3,403,238 of Sept.24, 1968 entitled Conversion of Heat Energy to Mechanical Energy, allpatents being assigned to the United States of America as represented bythe Secretary of the Navy. This material has a memory." If it is given afirst shape or configuration and subjected to an appropriate treatment,and thereafter its shape or configuration is changed, it will retainthat changed shape or configuration until such time as it is subjectedto a predetermined elevated temperature, for example, around F. When itis subjected to that temperature it tends quite strongly to return toits original shape or configuration. This material is available inelongated strip form. The strip can be given a bent configuration andheat treated, and can thereafter be straightened out to substantiallyliner configuration. It will retain that linear configuration untilsubjected to the predetermined activating temperature, at which time itwill revert to its bent configuration. Within a range of temperatures,the predetermined temperature at which the strip will tend to revert toits memory configuration can be varied by altering the proportions ofthe constituents of the intermetallic compound, as is known to the art.

A strip or other element formed of the material in question can beincorporated into an otherwise conventional thermostat construction, thestrip in its normal shape or configuration being relatively inactive onany of the parts of the thermostat. However, when the predeterminedactivating temperature is reached, which temperature is selected to besomewhat below the range of temperatures over which the thermostat isdesigned to be effective to actuate a switch, that strip will tend tochange its shape or configuration to its bent condition, in whichcondition it engages one of the switch elements, for example, the sameelement which is engaged by the bimetal, and tends to move that elementin the same direction as the bimetal is moving. Thus the tendency of thememory strip to return to its memory" configuration will be exerted inthe same sense as the force which the bimetal is exerting. The forcethus exerted by the memory" strip will be less than that needed toactuate the switch. Thus the memory" strip will not itself cause theswitch to be actuated. It will, however, greatly lighten the load on thebimetal, thus enabling the bimetal, over the operative adjusting rangeof the thermostat, to work against a much smaller force than haspreviously been thought necessary when the switch is to be actuated. Forexample, if the force of 1 118 ounces is thought desirable in order tomaintain the switch contacts in closed condition, the memory strip canbe designed so that when it is subjected to its activating temperatureit will exert a force on the switch contacts of 8-l0 ounces, andconsequently the bimetal, over the operating adjusting range of thethermostat, need work only against a spring force of from 38 ounces.

To the accomplishment of the above, and to such other objects as mayhereinafter appear, the present invention relates to a thermostatconstruction having an assist-action enabling the bimetal or othertemperature-sensing element to work against a minimal force over thecritical range of operation of the thermostat, all as defined in theappended claims and as described in this specification, taken togetherwith the accompanying drawings, in which:

FIG. 1 is a side elevational view of a typical stacktype thermostat inwhich the present invention is em-.

I ribs 18, and two .contacts 16 and 18 being located opposite oneanother. The spring leaf 8 is inherently resiliently biased downwardlytoward the spring leaf l0, and the spring leaf 10 is resilientlyinherently biased upwardly toward the spring leaf 8 with a forcesufficiently less than the downwardly biasing force on leaf8 so that thecontacts 16 and 18 are firmly pressed into engagement with one another.Threadedly mounted on the support 2 is an adjustment screw 20 whichpasses freely through an opening 22 in the spring leaf 8 and terminatesin a tip 24 against which the spring leaf 10 is adapted to be moved. Theparticular threaded location of the screw 20 will determine the limitingupwardly location of the spring leaf 10, since that leaf is alwaysbiased upwardly toward the screw tip 24. Under normal circumstances thespring leaf 8, being biased downwardly, will cause the contact 16carried thereby to engage the contact 18 carried by the spring leaf 10,the leaf 10 bending downwardly against its normal bias while the leaf 8bends downwardly relaxing its normal bias. This continues until theforces exerted by the leaves 8 and 10 equalize, the contacts 16 and 18then being urged firmly into engagement with one another. Terminals 26and 28 are electrically connected to the spring leaves 8 and 10respectively, and they may be connected to any suitable externalcircuit, such as a battery 30 or other source of power, a resistor 32which may be in form of a heater, and an on-off switch 34 adapted to bemanually controlled. When the switch 34 is closed, and when the springleaves 8 and 10 are free to assume their resiliently biased positions,the thus described circuit will be closed and the heater 32 will beenergized.

The bimetal 12 may be of the type which will bend upwardly as thetemperature to which it is subjected rises. It is provided with a finger36 which is adapted to engage the underside of the spring leaf 8 as thebimetal 12 bends upwardly, the spring leaf 8 extending out beyond thespring leaf 10 for this purpose. As the temperature to which the bimetal12 is subjected rises, the bimetal will tend to bend upwardly, and thattendency will be resisted by the inherent resilient biasing of thespring leaf 8, which is biased downwardly against the action of thebimetal 12. When the temperature to which the bimetal 12 becomessufficiently great, the bimetal l2, acting through the finger 36, willovercome the inherent downward bias of the spring leaf 8 and will liftthat spring leaf 8. The spring leaf 10 will move up wardly along withthe leaf 8, the contacts 16 and 18 remaining engaged, until the leaf 10engages the tip 24 of screw. Thereafter the leaf 10 cannot move anyfurther upwardly, but the leaf 8 will continue to be moved up by thebimetal 12. This will cause the contact 16 carried by the spring leaf 8to separate from the contact 18 carried by the spring leaf 10, thusopening the circuit to the resistor 32. The particular point in theprogressive bending of the bimetal 12 where the contact 16 will belifted from the contact 18 will be controlled by the temperature towhich the bimetal 12 is subjected. The higher the tip 24 of the screw20, the greater will be the temperature to which the bimetal 12 must besubjected before the contacts 16 and 18 are separated and the circuitcontrolled by the switch is opened.

All of the above describes the conventional thermostat. It will be notedthat in the operation of that thermostat the bimetal 12, in tending tomove to switchopening position, is always acting against the full forceof the spring bias on the spring leaf 8. If that bias is on the order of11-18 ounces, which is thought to be a desirable value in order toensure that the contacts 16 and 18 will engage one another with theforce and security necessary for the proper electrical operation of theswitch, then the bimetal 12 must, at the moment that it opens theswitch, be working against a force within that range. Moreover, theparticular force against which it will be working at the moment that theswitch is opened will vary somewhat depending upon the adjusted positionof the switch. When the screw 20 is screwed down, so that the switchwill be opened at the low end of the adjustment range, then the springforce exerted by the spring strip 8 and against which the bimetal 12operates will be relatively low, whereas if the screw 20 is screwed up,conditioning the thermostat to be actuated at a high temperature withinits range, the spring leaf 8 will be more distorted than in the previouscase, it will therefore exert a stronger downward force on the finger 36carried by the bimetal l2, and hence the bimetal will have to operateagainst a stronger force when the switch is to open at a hightemperature within its range of operation than when the switch isadjusted to operate in the low temperature area of its range ofoperation.

In order to minimize the force against which the bimetal 12 must operatewhen the switch is being opened or closed, and as specifically disclosedby the way of example in Fig. 1, an elongated bimetal-assist element 37is incorporated into the stack, that element being shown in the form ofa strip of appropriate material which extends out beyond the end of thespring leaf 8 and then is bent down, at 38, and up to 40, so as toengage the underside of the spring leaf 8. The element 37 is formed of amaterial having the characteristic previously described, to wit, thatwhen given a particular shape or configuration it will tend to maintainthat configuration until it is subjected to a predetermined temperature,and when that activating temperature is attained, the material will tendto assume a memory configuration different from its normalconfiguration. The strip 37 is designed to have an activatingtemperature slightly below the range of temperatures over which theparticular thermostat in question is designed to operate. For example,if the low point of the adjusting temperature range of the thermostat is100 F, then the strip 37 may be designed to have a critical activatingtemperature of perhaps 90 F. The strip 37 is shown in solid lines inFIG. 1 in its normal configuration. Its memory configuration, to whichit tends to return when it has been subjected to the activatingtemperature, is shown in broken lines in FIG. i. From the drawing itwill be seen that in its normal configuration the horizontal portion ofthe strip 37 is substantially straight line, whereas in its memoryconfiguration it tends to bend sharply at the line 42. When the strip 37is in its normal configuration as shown in broken lines its part 40 willnot exert any appreciable effect on the spring leaf 8. However, as maybe seen from the broken lines of the drawing, when the strip 37 tends toassume its memory configuration, it will tend to lift the spring leaf 8.The spring 37 is so designed as to produce a force, in tending to returnto its memory configuration, which is less than the minimum biasingforce exerted by the spring leaf 8, so that the strip 37, when it tendsto return to its memory configuration, is incapable of moving the leafspring 8 sufficiently to separate the contact 16 from the contact 18.However, since the force exerted by the strip 37 is in the samedirection as the force exerted by the bimetal 12 as it bends upwardlywith increased temperature, the bimetal 12, once the strip 37 tends toreturn to its memory configuration, no longer need act against theentire biasing force exerted by the spring strip 8. For

example, if, as has been stated, the normal biasing force is 11-18ounces, and if the force exerted by the strip in tending to return toits memory configuration is 8-10 ounces, then once the strip 37 has beentemperatureactuated the bimetal need work only against the force of 3-8ounces. This will make the bimetal considerably more accurate in openingand closing the switch in response to temperature changes. 1

When the temperature to which the thermostat is subjected falls and thebimetal 12 tends to bend downwardly, then the force exerted by thespring strip 8 on the strip 37 will tend to return that strip 37 to itsnormal condition or configuration, the strip 37 then being ready againto provide its bimetal'assisting function when its activatingtemperature is reached. In order to assist in restoring the strip 37 toits normal condition, its main length, up to the line 42 where it tendsto bend, may be supported on a rigid reinforcing strip 43 which is alsomounted in the stack. Both the strip 37 and the reinforcing strip 42, ifprovided, will have openings 46 and 46 respectively through which theadjusting screw 20 can freely pass.

FIG. 2 discloses another embodiment of the present invention, this timeusing a conventional switch 48 having terminals 50 and 52 extendingtherefrom, the switch being mounted in a casing 54 from which anoperating member 56 in the form of a button extends, there being aspring or other means within the housing 54 so as normally to urge thebutton 56 upwardly. When the button 56 is pushed in the status of theswitch 48 changes from closed to open or open to closed depending onwhether the switch is of the normally closed or normally open type. Inaccordance with the present invention; the switch dd is mounted on asupport 58. An arm 60 extends up from the support 53 and carries abimetal strip 62 which, for purposes of illustration, may be consideredas designed to bend downwardly progressively as the temperature to whichit is subjected increases. The arm 62 carries a finger 64 which engagesan arm 66 freely pivotally mounted at 68 on an arm 70 extending up fromthe support 58. The arm 66 in turn carries a depending finger 72 in linewith the button 56. A part 74 extending up from the support 58 carriesthe memory strip 76, the normal configuration of that strip being shownas substantially straight line and its memory configuration being shownin broken lines in FIG. 2, that memory configuration having the endportion of the strip 76 bend downwardly. As in the embodiment of FIG. 1,a strip 78 of rigid material may be provided immediately below the strip76 and extending up to the line 80 around which the strip 76 tends tobend when returning to its memory configuration.

In this embodiment the spring acting on the button 56 and tending tourge that button outwardly lifts the arm 66 and acts against the forceexerted by the bimetal 62 as the latter bends downwardly with increasein temperature. Thus under normal circum- I stances the bimetal 62 wouldhave to overcome all of the force which normally tends to urge thebutton 56 upwardly. However, in accordance with the present invention,the memory strip 76, when subjected to its activating temperature, andin tending to change its configuration to its memory state as shown inbroken lines in FIG. 2, will itself provide a force tending to overcomesome of the biasing force active on the button 56, and hence the bimetal62, at the temperature where it is designed to open the switch, need notact against the total biasing force on the button 56, but only on aforce equal to the difference between that total force and thebutton-depression-assisting force exerted by the strip 76.

It will be apparent from the above that the advantages of the instantinvention are achieved by means of structure which can be very readilyincorporated into existing thermostat designs, and with very littleincrease in cost over and above the cost of the memory element itself.Because that element does not exert any appreciable force on the switchover the normal inoperative temperature range of the thermostat, theswitch contacts are reliably and forcefully pressed against one another,thereby to produce low contact resistance and to resist shock andvibration. On the other hand, when the critical temperatures where thethermostat is to be operative are approached, the memory elementprovides an assist to the bimetal which permits the bimetal then tooperate against only a minimal opposing force and thus to operate morereliably and accurately than has previously been the case.

While but a limited number of embodiments of the present invention havebeen here specifically disclosed, it will be apparent that manyvariations may be made therein, all within the scope of the invention asdefined in the following claims.

Iclaim:

1. A thermostat comprising a switch means and an actuating elementoperatively connected to said switch means and effective on a change intemperature in a given direction to exert a progressively increasingactuation effect on said switch means, said switch means being active toresist said actuation effect to a predetermined degree and anactuation-assisting means operatively connected to said switch means,normally substantially ineffective to exert an actuation effect on saidswitch, but effective when a predetermined temperature is achieved closeto but somewhat below the desired actuation temperature for said switchto exert on said switch an actuation-aiding effect less than saidpredetermined degree but in the same sense as the actuation effectexerted by said actuating element and while said actuation effect isbeing exerted by said actuating member, whereby above said predeterminedtemperature said actuating element actuates said switch while exertingonly a minimal actuation effect thereon.

2. The combination of claim 1, in which said actuation-assisting meanscomprises an element formed 'of material having the characteristic thatover a given range of temperatures it will tend to retain a givenconfiguration but when subjected to a temperature beyond said range itwill tend to change its configuration, said element when in its normalconfiguration being substantially ineffective to exert said actuationeffect on said switch and when in its changed configuration beingeffective to exert said actuating-aiding effect on said switch.

3. The combination-of claim 2, in which said switch means comprises apair of arms, means biasing said arms toward one another, said arinscarrying engageable contacts, means for making electrical connection tosaid contacts, said actuating element comprising a bimetal elementcarrying a part engageable with one of said arms and active as thetemperature of said bimetal changes in a given sense to move said arm ina given direction away from the other arm, and adjustable stop meansengageable by said other arm and effective to limit its movement in saidgiven direction, said element comprising said actuation-assisting meanshaving a part located opposite and engageable with said one arm, saidelement when its given configuration so locating said part relative tosaid one arm as not to exert any appreciable operative force on said onearm, and when in its changed configuration so locating said part as toengage said one arm and move it in said given direction, the forceexerted by said element in tending to move from said given configurationto said changed configuration being less than that of said means biasingsaid one arm toward said other arm.

4. The combination of claim 3, in which said arms, said bimetal, andsaid element are mounted in a stack:

5. The combination of claim 3, in which said arms, said bimetal, andsaid element are mounted in a stack, said bimetal and said elementpressure-engaging said one arm on the same side thereof.

6. The combination of claim 3, in which said elemen comprises a sectionconnected to said part, said section in its said configuration beingsubstantially straight and said part then being located in the range ofmovement of said one arm corresponding to engagement between saidcontacts, said section in its said changed configuration assuming acondition bent in said given direction, thereby to move said part insaid given direction to a position beyond said range of movement of saidone arm.

7. The combination of claim 2, in which said switch means comprises anoperating member, means biasing said member to non-operating position,said bimetal element being operatively connected to said member andactive as its temperature changes in said given direction to move saidmember in a given direction to its operating position against the actionof said biasing means, said element comprising said actuation-assistingmeans having a part located opposite and engageable with said operatingmember, said strip when in its given configuration so locating said partrelative to said operating member and move it in said given direction,the force exerted by said element in tending to move from said givenconfiguration to said changed configuration being less than that of saidmeans biasing said operating member to non-actuating position.

8. The combination of claim 7, in which said element comprises a sectionconnected to said part, said section in its said given configurationbeing substantially straight and said part then being located in therange of movement of said operating member corresponding tonon-actuation of said switch means, said section in its changedconfiguration assuming a condition bent in said given direction, therebyto move said part in said given direction to a position corresponding toa switchactuating position of said operating means.

1. A thermostat comprising a switch means and an actuating elementoperatively connected to said switch means and effective on a change intemperature in a given direction to exert a progressively increasingactuation effect on said switch means, said switch means being active toresist said actuation effect to a predetermined degree and anactuation-assisting means operatively connected to said switch means,normally substantially ineffective to exert an actuation effect on saidswitch, but effective when a predetermined temperature is achieved closeto but somewhat below the desired actuation temperature for said switchto exert on said switch an actuation-aiding effect less than saidpredetermined degree but in the same sense as the actuation effectexerted by said actuating element and while said actuation effect isbeing exerted by said actuating member, whereby above said predeterminedtemperature said actuating element actuates said switch while exertingonly a minimal actuation effect thereon.
 1. A thermostat comprising aswitch means and an actuating element operatively connected to saidswitch means and effective on a change in temperature in a givendirection to exert a progressively increasing actuation effect on saidswitch means, said switch means being active to resist said actuationeffect to a predetermined degree and an actuation-assisting meansoperatively connected to said switch means, normally substantiallyineffective to exert an actuation effect on said switch, but effectivewhen a predetermined temperature is achieved close to but somewhat belowthe desired actuation temperature for said switch to exert on saidswitch an actuationaiding effect less than said predetermined degree butin the same sense as the actuation effect exerted by said actuatingelement and while said actuation effect is being exerted by saidactuating member, whereby above said predetermined temperature saidactuating element actuates said switch while exerting only a minimalactuation effect thereon.
 2. The combination of claim 1, in which saidactuation-assisting means comprises an element formed of material havingthe characteristic that over a given range of temperatures it will tendto retain a given configuration but when subjected to a temperaturebeyond said range it will tend to change its configuration, said elementwhen in its normal configuration being substantially ineffective toexert said actuation effect on said switch and when in its changedconfiguration being effective to exert said actuating-aiding effect onsaid switch.
 3. The combination of claim 2, in which said switch meanscomprises a pair of arms, means biasing said arms toward one another,said arms carrying engageable contacts, means for making electricalconnection to said contacts, said actuating element comprising a bimetalelement carrying a part engageable with one of said arms and active asthe temperature of said bimetal changes in a given sense to move saidarm in a given direction away from the other arm, and adjustable stopmeans engageable by said other arm and effective to limit its movementin said given direction, said element comprising saidactuation-assisting means having a part located opposite and engageablewith said one arm, said element when its given configuration so locatingsaid part relative to said one arm as not to exert any appreciableoperative force on said one arm, and when in its changed configurationso locating said part as to engage said one arm and move it in saidgiven direction, the force exerted by said element in tending to movefrom said given configuration to said changed configuration being lessthan that of said means biasing said one arm toward said other arm. 4.The combination of claim 3, in which said arms, said bimetal, and saidelement are mounted in a stack.
 5. The combination of claim 3, in whichsaid arms, said bimetal, and said element are mounted in a stack, saidbimetal and said element pressure-engaging said one arm on the same sidethereof.
 6. The combination of claim 3, in which said element comprisesa section connected to said part, said section in its said configurationbeing substantially straight and said part then being located in therange of movement of said one arm corresponding to engagement betweensaid contacts, said section in its said changed configuration assuming acondition bent in said given direction, thereby to move said part insaid given direction to a position beyond said range of movement of saidone arm.
 7. The cOmbination of claim 2, in which said switch meanscomprises an operating member, means biasing said member tonon-operating position, said bimetal element being operatively connectedto said member and active as its temperature changes in said givendirection to move said member in a given direction to its operatingposition against the action of said biasing means, said elementcomprising said actuation-assisting means having a part located oppositeand engageable with said operating member, said strip when in its givenconfiguration so locating said part relative to said operating memberand move it in said given direction, the force exerted by said elementin tending to move from said given configuration to said changedconfiguration being less than that of said means biasing said operatingmember to non-actuating position.